text: To generate a traveling pulse with an ultrashort time duration, two key elements are needed: bandwidth and central wavelength of the electromagnetic wave.
  From Fourier analysis, the more the available spectral bandwidth of a light pulse, the shorter, potentially, is its time duration.
  There is, however, a lower-limit in the minimum duration exploitable for a given pulse central wavelength. This limit is the optical cycle.
  Indeed, for a pulse centered in the low-frequency region, e.g. infrared (IR) 800 nm, its minimum time duration is around 2.67 fs, where  is the speed of light; whereas, for a light field with central wavelength in the extreme ultraviolet (XUV) at 30 nm the minimum duration is around 100 as.
  Thus, a smaller time duration requires the use of shorter, and more energetic wavelength, even down to the soft-X-ray (SXR) region.
  For this reason, standard techniques to create attosecond light pulses are based on radiation sources with broad spectral bandwidths and central wavelength located in the XUV-SXR range.
  The most common sources that fit these requirements are free-electron lasers (FEL) and high harmonic generation (HHG) setups.